NASA had already logged the two stars, which orbit in a way that each can eclipse the other from the Earth’s vantage point. However, it became apparent that the brightness of the system containing the two stars dimmed even when there was no eclipse. That suggested there was a third body in play.

Closer study showed that the dimming events took place at irregular intervals, meaning the two stars were not always in the same position, in turn meaning the third body was orbiting both stars. It also means the body experiences a double sunset at the end of its day.

Staff then looked at changes in the eclipse times of the two stars, which result from the gravitational pull of the third body. That was enough to give the mass, from which NASA has concluded the third body is a planet roughly the size of Saturn; given the location its likely split fairly evenly between rock and gas.

Though several bodies orbiting two stars, known as circumbinary orbit, have been identified before, this is the first time there’s been confirmation of such a body that NASA would define as a planet. Unfortunately while Kepler is examining an area of the Milky Way that’s most likely for liquid water on a planet’s surface to be possible — the theoretical key to supporting life — the two stars are too cool and Kepler-16b too far away from them for this to be possible in this case.